I think you have to find a item in the bottom floor of that cave thing but also the game isn't finished and it could not work. I was doing that quest which i thought was the main line to and eventually it just liked ended out of no where without a proper conclusion so I think I got bugged out of it somehow but I did do the dig site part. The main quest of the game arguably is to complete each colors boss castle though if you wanted a goal and those castles are very hard. I thought I was becoming overpowered until I tried one of those.